1. Field of the Invention
This invention relates to a multi-substrate package and method for assembling same
2. Background Art
Previous research on manufacturing dense packages involved various multi-chip module technologies, including chip scale packaging using solder, wire bonding, flex substrates, epoxy layers, filled vias, microrelays, and ceramic clusters.
The following publications show such technologies:                1. S. F. Al-Sarawi et al., “A Review of 3-D Packaging Technology,” IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY, 21, No. 1, pp. 2-14, February 1998;        2. M. Schuenemann et al., “MEMS Modular Packaging and Interfaces,” PROC. OF THE 50 TH ELECTRONIC COMPONENTS AND TECHNOLOGY CONFERENCE, pp. 681-688, 2000;        3. D. C. Miller et al., “Micro-relay Packaging Technology Using Flip-Chip Assembly,” PROC. OF THE 13 TH INTERNATIONAL CONFERENCE ON MICRO-ELECTRO-MECHANICAL SYSTEMS, pp. 265-270, Miyazaki, Japan, Jan. 23-27, 2000;        4. A. Mason et al., “A Generic Multi-Element Microsystem for Portable Wireless Applications,” PROC. OF THE IEEE, PP. 1733-1746, August, 1998;        5. H. Goldstein, “Packages Go Vertical,” IEEE SPECTRUM, pp. 46-51, August, 2001;        6. K. D. Gann, “Neo-Stacking Technology,” HDI MAGAZINE, December, 1999;        7. M. S. Bakir et al., “Sea of Leads (SoL) Ultrahigh Density Wafer-Level Chip Input/Output Interconnections for Gigascale Integration (GSI),” IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. 50, No. 10, October, 2003; and        8. B. Murali, “Bridging the Gap Between the Digital and Real Worlds: The Expanding Rule of Analog Interface Technologies,” 2003 IEEE INTERNATIONAL SOLID-STATE CIRCUITS CONFERENCE, Plenary Session 1.3, Feb. 10, 2003.        
U.S. Pat. No. 5,701,233 discloses stackable modules and multi-modular assemblies.
However, of the above prior art, none provides the flexibility, modularity, and small size needed for a microsystem containing substrates with sensors, actuators, and circuits fabricated using different technologies.
Modularity and reworkability are particularly important in multi-substrate packages typically encountered in microsystems. Although it is possible to use completely disposable systems and throw out the entire system when it malfunctions (either in-situ after it is in the field, or after the completion of the manufacturing process), it is often desirable to have some level of reworkability to be able to replace any given die in a multi-substrate package. Already operational dice can be lost when, for example, the system package is being repaired to replace other malfunctioning dice. Replacement of these defective dice is a complex procedure, which may result in loss of electrical connection during the removal process. Inhibited access to the component pad site due to the high component population densities is another problem. Reworking is not practical in a hard-wired system.
Reworkability of microsystems with dice containing MEMS devices is yet more difficult, because these devices often contain fragile structures, and assembly and reworking becomes even more challenging. However, in most microsystems applications, the number of output pads needed for signal transfer is fewer than what is typically needed in VLSI circuit chips, which typically require hundreds of I/Os per chip. For a die with MEMS structures, it is usually the structure size, not the pad size that defines the die size, so the minimum pitch and pad size can be on the order of 100s of microns compared to IC's minimum pad size required of 50-100 μm.
Although these specifications seem to make microsystems with MEMS devices easier to assemble, there are additional assembly and packaging challenges involved with such microsystems. High quality signal transfer from the sensor to its readout circuitry is important for increasing the resolution and dynamic range of the system. Decreasing the length of interconnection lines and eliminating large-scale connections between dice are required for better system performance. This is also the motivation behind much of the research in various areas of microsystems to integrate sensors and actuators with electronics. Moreover, due to the large variety of MEMS devices, not only electrical connections, but also fluidic and/or optical connections are needed between dice of a microsystem.
In the case of fluidic connections, chemical reaction between the fluidic connector material and the sample fluid flowing in the fluidic channels, hermeticity of the fluidic channel itself, and hermeticity of the seal between the connector and the inlet on the MEMS sensor are some of the factors that determine the quality of fluidic connections. For a high-sensitivity gas sensing system, for example, even the smallest amount of gas is important, and should not be lost due to leakage in the connector path.